a) Field of the Invention
The invention is directed to a burner with a housing having a combustion chamber with an inlet for a gas/air fuel mixture and an outlet for the exhaust gas.
b) Description of the Related Art
Conventionally, burners of this type work with a free-burning flame burning the gas/air mixture in the combustion chamber and the hot combustion gas is used as a heat source. In particular, the hot combustion gas is guided past water-carrying pipes for heat transfer so that hot water or steam is generated in these pipes.
Pollutants such as NO.sub.x and CO are formed in such burners. These toxic and health-threatening gases occur at high flame temperatures, by incomplete combustion in unstable flames or at a lower flame temperature which could indeed be reduced, but only at the expense of an unstable flame. Further, incomplete combustion of the gas/air mixture must also be expected, resulting in reduced efficiency.
Various types of burner have been developed to overcome these disadvantages. A survey appears in "Lean-Burn Premixed Combustion in Gas Turbine Combusters", A. Saul and D. Altemark, Vulkan-Verlag, Essen, vol. 40 (1991), 7-8, pages 336-342. The substantial feature in the developments described in this literature for reducing pollutants is primarily a low flame temperature and various steps are undertaken to burn the fuels as completely as possible. The most important measures for achieving a more efficient combustion are superstoichiometry and catalysis. For example, the cited literature indicates a rich-quench-lean combustion chamber, model "LM 2500" by General Electric, which is still in the developmental stage, in which a rich fuel mixture is burned in a first step. In an intermediate step, air is supplied to the gas which has been partially burned in the first step and the resulting lean mixture is burned in a second step. The authors indicate a NO.sub.x content in the gas of less than 190 mg/m.sup.3 for this burner.
The literature cited above also describes combustion by catalysts, by means of which complete combustion can be achieved at a low temperature. The literature indicates a NO.sub.x content of less than 20 mg/m.sup.3 for catalytic combustion. Catalytic combustion is in development at a number of research facilities but has not yet progressed beyond the research stage. In the opinion of the authors, it is not expected that this type of burner will be used commercially within the next five years.
Problems of stability are not discussed in detail in the cited literature. However, such problems increase in importance the lower the selected flame temperature.
A possibility for stable combustion at low temperatures is discussed in "New Gas Burner and Gas Burner Equipment Technology", a contribution of the gas industry to environmental protection, Otto Menzel, gwf Gas/Erdgas 130, 1989, No. 7, pages 335-364, and in "Development of a low-pollutant premix burner for use in domestic gas heat boilers with cylindrical combustion chambers", H. Berg and Th. Jannemann, Gas Waarme International, vol. 38 (1989), No. 1, pages 28-34, Vulkan-Verlag, Essen. The "Thennomax" burner described therein has only a low NO.sub.x output. The flame stability in this burner is achieved by means of a heat-conducting burner plate substantially comprising a perforated plate with round bore holes, the gas to be burned flowing through these holes. Due to the elimination of heat via the perforated plate, the flame is practically held to the burner plate resulting in a stable flame.
However, the burner plate is also not a satisfactory assurance of flame stability under all operating parameters. It is stated, for example, that preheating of the mixture to approximately 300.degree. C. should be carded out at high air ratios since this increases the combustion rate and accordingly reduces the lift-up tendency of the flames.
It is clear from the cited prior art that it is possible to achieve a reduction in pollutants by means of lower flame temperature, but the stability of the flame still poses an important unsolved problem.